Water treatment systems such as seawater desalination facilities and osmotic power plants (facilities that generate energy from a difference in osmotic pressures of e.g. seawater versus river water) include an intake unit for delivering water from its source (e.g. sea or river) to the system, and a pretreatment unit for removing floating and dissolved material from the delivered water, in order to prepare the water for the main membrane process.
Two types of intake units are open intakes and infiltration intakes (or infiltration galleries). Open intakes draw water via piping directly from the source. Open intakes typically employ screen meshes to filter out large debris and prevent fish or other marine life from being drawn into the pumps. However, millions of fish and other small marine organisms, with a width of under 2 cm are sucked into the piping, leading to considerable damage, both to the environment and to facilities. Damage is inflicted on both large aquatic organisms such as fish or crabs that are trapped against the intake screens and drown or suffocate, and on small marine organisms such as fish, fish eggs, larvae or plankton that is drawn into the intake system and is killed by the plant equipment.
Infiltration intakes, or galleries, are built in the seabed by the installation of horizontal drain systems. The drain system is placed in the natural filtration media sand, and the seawater is slowly filtered by the sand. This media is naturally cleaned by waves and storms. Horizontal drain systems deliver water to the pumping station located on the seashore. Infiltration galleries, while protecting the marine environment, can only be installed in areas with naturally occurring sands. Another major limitation is that these systems clog over time and it is highly difficult, or in some cases impossible, to clean them. Clogged media reduces the throughput through the system by two orders of magnitude (e.g. from 10 to 0.1 m3/hr).
Pretreatment units employ a layer of filter media supported by a drainage layer. Water is introduced above the filter media, and is pretreated by flowing through the filter media which removes floating and dissolved material therefrom. The filter media is gradually clogged by the removed material, and periodical global backwashing is used to clean the filter media. Global back washing produces huge amounts of wastewater which leads to environmental and technical problems. The backwashing process also involves interrupting the operation of the filter, and this is a major drawback too.
Such a global backwashing system according to the prior art is illustrated in FIG. 1A. FIG. 1A illustrates a prior art filter cleaning method for back washing a filter 90 that is used to filter water 91 through filter media 92 into a drainage layer 96 (under-drain) that supports filter media 92. The filter cleaning method uses an external source of backwash water that is pumped throughout the whole filter 90 to backwash the filter globally (see arrows). The backwash water is then removed gravitationally through a discharge channel 132. The large volume of backwash water requires operation of the filter with a high level of water 91 above filter media 92 (denoted in FIG. 1A by H) to allow for expansion of the filter media, during which sludge is released from the filter media particles. The necessarily large water head has severe constructional implications, as the substrate must support the large pressures. Hence, prior art backwashing systems suffer from a severe limitation. Clearly, this method of backwashing is not applicable to infiltration intakes as they are open to the water source (such as a sea or a river) and contamination of the source with the backwash water is hardly acceptable.
U.S. Pat. No. 4,486,307 (Weiler) describes a filter apparatus having filter bed back-washing means comprising a raisable and lowerable suction bell, the side walls of which define division walls to compart the respective partial volume of the filter bed to be cleaned. The suction bell is mounted on a bridge and the bell is raised and lowered by a lifting device. The bell is connected to a suction pump mounted on the bridge. In order to clean the filter bed of suspended matter filtered out of the crude water to be purified, the suction bell is lowered into an operative position. In order to facilitate the penetration of the lower edges of the side walls of the bell, the walls are provided with sharp stabbing edges and a vibrator is arranged on the top side of the suction bell. The suction action sucks water out of the filtrate chamber to make the bottom permeable to liquids. As the partial volume of the filter bed located in the suction bell is washed, the sand of the filter bed is loosened and eddied whereby the suspended matter retained in the respective partial volume is loosened and exhausted by the suction action of the pump.
U.S. Pat. No. 4,988,439 (Medders) relates to liquid filtration systems having a travelling bridge-type cleaning apparatus for sequentially cleaning a plurality of suspended solids filter units. The carriage means carries a hood assembly enclosing air scour means and liquid backwash means, the hood assembly being provided with sealing means to establish a substantially air and wafer tight seal between the hood and a filter cell.
While these systems are satisfactory, backwashing of a sand bed used as the filter media suffers for the severe drawback of obtaining sufficient penetration of the compacted sand bed to obtain adequate filtering of the water through the filter media.
It is an object of the present invention to provide an improved method and apparatus for backwashing a filtering system that aims to overcome, or at least alleviate, the abovementioned drawbacks.